The mechanism of the total synthesis of acetate from CO2 by Clostridium thermoaceticum and by Clostridium acidiurici is being investigated. The synthesis by C. thermoaceticum involves transmethylation from methyltetrahydrofolate to the cobalt of a corrinoid enzyme and carboxylation of the resulting methyl corrinoid by transcarboxylation from pyruvate. Five fractions have been obtained which, when combined, catalyze the synthesis of acetate from pyruvate and CH3-tetrahydrofolate. The role of each of these components in the overall reaction is under study. C. acidiurici synthesizes acetate from CO2 by a non-corrinoid pathway. This synthesis involves glycine decarboxylase and the mechanism of this synthesis is being investigated.